Within the Earth's geological formations, oil and gas deposits are relatively rare. In order to form oil and gas deposits, the geological formations need to include anticlines for storing organic material in a stable manner for long periods of time. Moreover, specific conditions of temperature and pressure need to prevail during a period of many millions of years for the oil and gas deposits to form in the anticlines. If these specific conditions are not satisfied, either coal is formed or alternatively no hydrocarbon deposits are formed.
Present methods of performing metrology within geological formations are based upon seismic measurements and electromagnetic measurements. Land-based seismic measurements involve deploying seismic sensors over a region of the Earth's surface in contact with the Earth, and then detonating an explosive charge to create seismic shockwaves within the Earth. The shock waves propagate to a geological formation and are at least partially reflected by acoustic impedance mismatches at interfaces between mutually different rock layers in the formation. Reflected seismic shock waves are subsequently received at the seismic sensors to generate signals which are processed to generate a seismic image of the geological formation. Although the seismic image is able to provide information regarding regions of rock, it is often not a reliable method of ascertaining whether or not hydrocarbon deposits such as oil or gas are present in a given anticline. Seismic investigations are often complemented by expensive drilling activities which, in certain situations, yield a negative result of no oil or gas being present.
When exploring for oil in off-shore locations, mounting seismic sensors onto a sea-bed region and then detonating an explosive charge within the sea-bed region is a time-consuming and expensive activity. Moreover, drilling activities at off-shore locations are considerably more expensive than on land locations, placing an even greater burden on initial seismic measurements providing a representative indication whether or not oil and/or gas deposits are present.
Such technical difficulties and associated high costs for performing off-shore drilling have prompted development of alternative techniques for finding subterranean oil and/or gas deposits. For example, in a presently pending Norwegian patent application no. 20053085, there is described an electromagnetic method of investigating a region which potentially includes a subterranean hydrocarbon reservoir. The method employs an apparatus which is submerged in operation and towed near a geological region to be investigated. The method involves launching from the apparatus an electromagnetic interrogation signal into the geological region, receiving complex multipath reflected signals from the region in response to the interrogation signal being received at the geological region, resolving the reflected signals into at least two orthogonal spatial axes and then comparing a phase relationship of the received reflected signals along these two axes for determining whether or not hydrocarbon is present. The apparatus employs a single horizontal dipole antenna driven by a signal generator. Other granted Norwegian patents, for example Norwegian patent no. 325116 discloses use of electromagnetic waves for determining a presence of subterranean hydrocarbon reservoirs. Such methods are conveniently referred as being “Controlled Source Electro Magnetic” (CSEM).
A problem encountered is that these apparatus operable to be towed in a submerged state for generating relatively-high intensity electromagnetic radiation is that they are costly to build and deploy. Moreover, their electromagnetic radiation output power is limited by employing signal generators for driving transmitting dipole antennae.
For example, there has earlier been employed controlled electromagnetic sources for driving antennae of these aforementioned apparatus, for example sources operable to output formed square-wave signals, saw-tooth signals, and sinusoidal and modulated sinusoidal signals. Presently known methods of employing these formed signals suffer a problem that the output signals for driving antennae are insufficient in amplitude and have insufficient temporal sharpness, namely insufficiently fast rise-time, to enable sufficiently clear processing and analysis of reflected signals to determine with certainty whether or nor oil and/or gas deposits are present in a subterranean region.
The present invention therefore seeks to provide more cost effective and simpler methods of measuring geological formations using electromagnetic radiation.